Seismic images of a given area are generated by introducing a seismic wave into the area from a source. A portion of the wave proceeds downward, into the ground, until a portion thereof is reflected back upward by a “reflector” due to impedance changes, according to Snell's law. The reflected wave generally proceeds upwards and is detected by geophones or other receivers disposed at mapped locations, offset from the introduction point of the wave. The signals from the various receivers over time are then “stacked” or added together, to form a more complete seismic image. The stacking process may include a variety of analysis techniques, including moveout analysis, to compensate for the different locations of the receivers.
Moreover, prior to stacking, the pre-stack signals may be filtered to remove noise therefrom, which may result in a crisper image. Multiple reflections, or “multiples,” are one type of noise that is to be filtered. Multiples occur as a returning, reflected signal encounters a boundary between two rock layers of different impedance. As with the downward moving signal, the reflected signal is partially reflected, but this time, back downwards. Accordingly, the signal may reverberate between boundaries with relatively high impedance contrast ratios several times, before returning to the surface. As such, the time at which the signal is received may be inaccurate and may produce interference with primary signals of deeper layers.
In many cases, multiples may be non-hyperbolic events, in contrast to the hyperbolic primary signals. Thus, the multiples may be sufficiently distinct from the primary signals that they too may be filtered out during pre-stack processing, using any one or more of a variety of processes. However, in areas where a high dip in the seismic profile is seen below a strong reflector, high-order multiples may be present, such as “peg-leg” multiples. Peg-leg multiples are multiples that traverse a non-symmetric path from the introduction point to the receiver, i.e., may reflect upward off of a first reflector, then downward off of a second reflector, then upward again off of a third reflector at a different level from the first, etc. Peg-leg multiples may manifest as near-hyperbolic events. Accordingly, at least portions of the peg-leg multiple may closely mimic the primary signals from lower levels, such that the peg-leg multiples are difficult to filter out in pre-stack processing and thus may be included in the stacked image.